Aerial navigation seeks to avoid turbulent atmospheric zones. To detect them and anticipate them, aircraft are generally furnished with a radar, the antenna of which scans and illuminates the regions through which the carrier aircraft is liable to pass. The reception and processing of the meteorological echoes, after eliminating echoes of another nature such as ground echoes for example, provide the variance of the radial velocity field of the wind, more particularly of the moment of radial velocity of order 2.
In reality, the velocity of the wind is not measured directly, but rather the velocity of tracers entrained by the motions of the air mass is measured. These tracers are, more often than not, hydrometeors such as drops of water, snow, hail or supercooled ice for example. Near the ground, it is also possible to utilize the reflection of the radar waves on non-aqueous meteors, such as dust or insects for example, entrained by atmospheric motions. Nevertheless at high altitude, only hydrometeors are usable.
A meteorological radar first identifies the dangerous zones through a reflectivity measurement. Indeed, high reflectivity corresponds to significant precipitations and therefore to dangerous convective phenomena. Conversely there exist situations from the point of view of aerial navigation which are not necessarily associated with a high reflectivity. Such situations are encountered notably:                in the vicinity of convective systems where dangerous air motions may be present without significant precipitations;        because hydrometeors present at high altitude are more often than not frozen and therefore weakly visible to a conventional meteorological radar.        
Moreover, it is desirable that the crew of an aircraft be afforded the earliest possible alert so as to reroute the aircraft or to take the necessary measures to guarantee the safety of the passengers on board.
One problem is related to aircraft carriage constraints. Indeed, the emission power and the dimension of aerial equipment being limited by these carriage constraints, it is consequently necessary for measurement algorithms to be as sensitive as possible.
Another problem stems from the fact that knowledge of the variance of the wind field velocity is not alone sufficient to measure the degree of severity of the effect of turbulence on an aircraft. Indeed for a given variance the effect on the airplane, measured in terms of load factor, depends on the way in which this turbulent velocity field varies over time when the aircraft passes through it. Stated otherwise, fine characterization of the impact of turbulence on an aircraft requires the measurement of the spatial autocorrelation function of the turbulent velocity field.